The present invention relates to motor controls and more particularly to a two current sensor assembly for determining three phase current values and for providing over-current protection.
Many motors include three phase windings driven by a controller which provides three phase AC currents to the three windings via three separate AC supply lines. To provide controlled operation, many control systems include closed loop current feedback so that motor operation can be monitored and adjusted accordingly.
In addition, many systems come equipped with over-current protection circuitry to minimize the effects of short circuited supply lines. To this end, over-current protection circuitry typically includes a means for monitoring all three line currents and means for shutting off the line currents when a current irregularity is identified. When one of the line currents exceeds a predetermined threshold value, the circuitry recognizes the possibility of a short and shuts off current to all three motor phases effectively stopping the motor until the cause of the irregularity is identified.
The easiest way to provide current feedback for both motor control and over-current protection is to provide three current sensors, a separate sensor on each of the three supply lines. Currents sensed via the three sensors is provided to the over-current protection circuitry and the controller generally to facilitate over-current protection and motor control, respectively.
Typical current sensors used in the motor controls art include Hall effect sensors. A Hall effect sensor typically forms a passageway through which one or more lines can be passed. The sensor senses the total current passing through the passageway and provides an electrical signal indicative thereof.
While the three sensor solution is robust and easy to understand, unfortunately current sensors are relatively expensive. Therefore, it is desirable to use fewer than three sensors when possible.
To this end, some systems have been designed which are capable of measuring two line currents and deriving the third line current therefrom. It is well known that the sum of currents passing through the three supply lines should equal zero. Thus, if the first and second line currents are sensed, during normal operation the third line current can be determined by taking the negative of the sum of the first and second currents. Here, all three line currents can be derived for control purposes using only two Hall effect sensors.
Unfortunately, where only two line currents are measured and the third current is derived, the derived current may not reflect a fault to ground in the third line. This is because a connection to ground in the third line with a relatively high impedance may not significantly affect the currents in the first and second lines.
If a short occurs in a third line and is not detected because the third line current is derived via the first and second sensed currents, the over-current circuitry cannot operate properly to shut off current to the three phases and motor damage or operator injury may result.
Thus, it would be advantageous to have a current sensing apparatus wherein all currents in a three phase motor system can be derived using less than three current sensors and complete over-current protection can also be provided via the sensed currents.